I agree with jeanie, buffalo is going to be a tad on the lean side so it may not hold up well for the stuffing. But it's good stuff. A bit more gamey flavor than beef, though not overpowering. I happen to like it. There's a buffalo ranch about a mile or so down the way from me so I get it often. A good buffalo t-bone on the grill can't be beat!
But as far as a fatty goes, I'd add something with a little more fat to help it hold together, or maybe mix in some breadcrumbs and egg as a binder like you would for a meatloaf. I did that for some 90/10 beef that I used for a fattie and it made it easier to mold. Also be sure to put it in the freezer until it's just slightly frozen, that'll help it hold together too.
For stuffing, like Jeanie said, just about anything would be good. I think your idea of cheese, mushrooms and onion sounds great.